A strong explosive eruption is underway at Okmok Volcano on Umnak Island in the eastern Aleutians. The volcano is currently at aviation color code RED and alert level WARNING. All areas immediately around the volcano are considered hazardous. Airborne ash and gas continues to drift with the wind and pose a hazard to aviation in the area. Additional ash fall will occur on Umnak Island and possibly adjacent islands as long as the eruption continues.

Observations and Background

The eruption began abruptly at 11:43 AM on July 12 after about an hour of rapidly escalating earthquake activity.

Ash and gas from the initial explosions reached at least 50,000 feet above sea level on July 12 and drifted as a large cloud south and east where it remains in the atmosphere above the North Pacific. Satellite tracking of the ash cloud by traditional techniques has been hampered by the high water content. Heavy ash fall occurred on the eastern portion of Umnak Island; a dusting of ash also occurred for several hours in Unalaska/Dutch Harbor.

At this time, based on AVO analysis of satellite data, ash is continuing to erupt from a composite cinder and spatter cone called Cone D in the eastern portion of the 6-mile wide caldera or crater of Okmok. It appears that the eruption is very water-rich due to interaction of rising magma with very shallow groundwater and surficial water inside the caldera. We have few direct observations into the caldera and details of the current event remain unknown.

The current activity differs in character from the past three significant eruptions at Okmok in 1945, 1958, and 1997. All of these eruptions occurred at a cinder and spatter cone on the far western portion of the caldera floor, Cone A. In general, each eruption was mildly to moderately explosive with most ash clouds produced rising less than 30,000 ft above sea level. Each eruption also produced a lava flow that traveled about 5 miles across the caldera floor.

Prognosis and hazards

Based on past eruptions at Okmok and our analysis of the current episode of activity, we would expect this event to continue for several weeks and possibly longer. The position of the vent in the eastern caldera adjacent to a shallow lake suggests that water will continue to play a role in increasing the explosivity of the eruption resulting in significant ash and steam production. If the eruption follows patterns of previous Okmok events, lava will eventually reach the surface to form lava fountains, spatter accumulations, and possibly a lava flow. It is also possible that explosivity could intensify at any time.

Conditions around Okmok volcano on Umnak Island remain hazardous at this time. Ash fall is expected to continue downwind of the volcano including over marine areas in the North Pacific. The caldera and areas in the immediate vicinity of the volcano on Umnak Island should be avoided, particularly the Crater Creek drainage northeast of the caldera. Strong explosions are likely producing ballistics or larger particles of tephra that can impact the area around the caldera rim and surrounding area for several miles.

Pyroclastic flows and surges, dangerous fast moving clouds of ash, larger particles, and hot gas, could form and travel across the caldera floor as well as over the caldera rim and down slope. Mudflows or lahars may form as rain mixes with the ash fallout. These will likely be channeled down drainages from the caldera rim, especially to the east and south of the volcano, including Crater Creek which drains to the Bering Sea to the northeast of the caldera. Depending on the evolution of activity within the caldera, Crater Creek may become dammed posing an extreme flooding risk in the Crater Creek Drainage.

AVO will continue to monitor the volcano closely. The operations room at AVO in Anchorage is now staffed 24 hours per day. We will attempt to visit the volcano as soon as conditions allow in order to better document and understand the activity and ongoing hazards. New data and observations may lead us to change our assessment. Any changes would be announced in a subsequent Volcanic Activity Notice. AVO appreciates the cooperation and assistance of mariners, pilots, residents of Unalaska and other Aleutian communities who are sending observations, ash samples, and photographs via our web site.

Further information on Okmok Volcano and related hazards can be found at the following web sites:

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.

VOLCANO ALERT LEVELS

NORMALVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

ADVISORYVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

WATCHVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway but poses limited hazards.

WARNINGHazardous eruption is imminent, underway, or suspected.

AVIATION COLOR CODES

GREENVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

YELLOWVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

ORANGEVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway with no or minor volcanic-ash emissions [ash-plume height specified, if possible].

REDEruption is imminent with significant emission of volcanic ash into the atmosphere likely OR eruption is underway or suspected with significant emission of volcanic ash into the atmosphere [ash-plume height specified, if possible].